How do baggage scanners at the airport work? What do the different colors on the x-ray mean?

The x-ray devices in luggage scanners are a bit more complicated than the x-ray medical imaging devices. In particular, they're set up to identify and distinguish between different kinds of objects, whereas medical x-rays are pretty much only interested in one: bones.

The trick is that not all x-rays are created equal. The x-ray source will send out x-rays in a range of energy levels. Organic objects block some low-energy x-rays, but not higher-energy x-rays. Plastics block low-energy x-rays better than organic objects, but not as effectively as metal objects, which pretty much block everything.

The device is set up so that the x-rays pass through your luggage and then hit the first detector, which sends an image to the computer. But the x-rays then pass through a filter that blocks out all low-energy x-rays before hitting a second detector. That sends another image to the computer, this time only showing those objects that block high-energy x-rays. By combining these two images, the software can distinguish between organics, inorganics (e.g., plastics), and metals. It then assigns different colors to each.

Most manufacturers use black for metal and orange for organics, but it could vary from machine to machine.

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